(60c) Core-Shell Nanostructured Anodes for Lithium Ion Batteries

In the last ten years there have been great advances in the production technologies of Lithium Ion batteries, making them one of the most popular sources for portable computing and telecommunication equipment. In particular, a lot of work was done on the electrochemical properties of electrode Materials for Lithium Ion Batteries using Graphene (G) and Graphene Oxide (GO) with silicon, however, these works suffer from several drawbacks associated with the breakage of the Solid Electrolyte Interface (SEI) during repeated cycling. In this work we developed a new method that avoids breakage of the SEI at the silicon surface. We first created novel core shell/hollow nanocarriers made with GO (*) and then incorporated silicon as the high capacity electrochemically active material which has the potential to greatly increase the capacity of these materials.

The ultrathin graphene oxide membranes encapsulate the nanoparticles with controlled void space, allowing for the expansion and contraction of the silicon as it is alloyed with lithium during cycling of the battery. Under the correct processing conditions the graphene oxide was almost completely templated from the aqueous phase to form ultrathin membranes about the silicon nanoparticles. We used a modified emulsion precipitation method to encapsulate silicon nanoparticles. Because this is fundamentally a directed self-assembly process, it was possible to achieve control over the size of the resulting droplets of the emulsion. It is a directed self-assembled process in the sense, that although the membrane of GO spontaneously forms upon the interface between the oil phase and aqueous phase, energy input (e.g., sonication, rotor-stator, high pressure homogenization, or energetic stirring) is required to finely divide the phases and create the interfacial area. The graphene oxide provided high stability to the emulsions, on the order of minutes or hours. The emulsions could be stabilized down to graphene oxide concentrations as small as 40 ppm. Increasing the graphene oxide concentrations above 40 ppm enhances the stability of the emulsions.

We used different size of graphene oxide sheets to develop the process for different size emulsion particles. By purposely using relatively large graphene oxide sheet (relative to the size of the particles) we ensured that the graphene oxide did not become overly hydrophilic. The reason for this is because GO sheets tend to have carboxylic acid groups in the edges, so the smaller the length, the more edges they have and the more hydrophilic the GO sheets become. In order to have the GO sheets attract to each other, that situation is not favorable because it affects the stacking and plating of the sheets. By increasing the sheet size, the sheets become larger in length and therefore the number of edges per unit area decreases, and hence, the hydrophilic groups decrease so the sheets become more hydrophobic and attract to each other forming thicker membranes (plating). We present the analysis of the graphene oxide sheet size distribution and mean sheet sizes use in the synthesis of the core-shell particles. The composite structures produced by this method were thermally reduced to convert GO to a graphitic structure capable of providing high electrical conductivity as well as high lithium ion conductivity. We solidified the composite to effectively trap the electrochemically active silicon nanoparticles within the matrix, preventing segregation of the active components from the conductive matrix. Our results show anodes cycled at very high charge and discharge rates of 3A/g are possible. We varied the percent of silicon inside the graphene oxide structures, and we report capacities starting at 2000 mA-hr/g. We present the results of both thermally reduced and chemically reduced graphene oxide based anodes. Our particles were characterized via laser diffraction, zeta potential, FE-SEM, TEM, BET, and AFM.

(*) Langmuir March 2017. Kurt B. Smith, Maria S. Tomassone, “Ultrathin Hollow Graphene Oxide Membranes for Use as Nanoparticle Carriers”

DOI: 10.1021/acs.langmuir.6b04583